The proposed study will attempt to identify the neurotransmitter(s) which subserve excitatory transmission at auditory nerve synapses. Putative excitatory neurotransmitters will be iontophoretically applied into the extracellular space immediately surrounding single neurons in the various regions of the cochlear nucleus (CN). Recent studies strongly suggest glutamate and aspartate as transmitters at the junction of the cochlear portion of the statoacoustic nerve and the cells of the cochlear nucleus (Wenthold and Gulley, 1977; Bird, 1978; Wenthold, 1978). Microneurochemical studies indicate that L-glutamic acid and L-aspartic acid show a regional distribution within the cochlear nuclear complex (Godfrey et al., 1977b; 1978) which resembles the distribution of primary afferents. Iontophoretic application of glutamate, aspartate, and their agonists, kainic acid and N-methyl-D-aspartate may therefore have differential effects on neurons in the different areas of the cochlear nuclei. Preliminary results in this laboratory indicate that iontophoretic application of glutamate, aspartate and kainic acid can excite neurons in the CN, increasing their responsiveness to auditory stimuli. Certain neurons in the CN respond to iontophoretic application of these substances by: 1) changes in best-frequency threshold to a stimulus tone; 2) profound changes in spontaneous activity. It should be possible, by carefully mapping responses to glutamate, aspartate, and other putative excitant neurotransmitters and their agonists, to generate a map of the distribution and efficacy of the excitatory responses to these substances of neurons in the cochlear nuclei. It is important to understand neurochemical aspects of sensory processing since it is conceivable that a number of neurologic abnormalities such as receptive aphasia and "auditory perceptual dysfunction" may involve a neurochemical fault. The combination of neurophysiologic and neuropharmacologic techniques should aid in identification of auditory neurotransmitters and elucidation of their role in the coding process.